I installed CentOS with minimal configuration and installed ProFTPD from Dag Wieers RPM repository for APT and still could not get it working. As soon as I continued with the install (following this guide - http://www.howtoforge.com/forums/showthread.php?t=470), ProFTPD worked.

What changes does ISPConfig make to get ProFTPD to work? Or what else is installed in the setup to get ProFTPD to authenticate?